1. Field of the Invention
The present invention relates to a process for producing organic compounds such as lower alkyl alcohols, including ethanol, propanol (e.g. 1-propanol, iso-propanol), and butanol (e.g. 1-butanol), from gases including carbon dioxide, carbon monoxide, and hydrogen under thermodynamically favorable conditions; microorganisms used in the process to produce organic compounds from gases; and a process for enriching, isolating, and improving microorganisms that can be used in the process to produce organic compounds from gases. The process may also be used to produce one or more carboxylic acids including acetic acid, propionic acid, or butyric acid, other carboxylic acids, especially longer carboxylic acids, and the process produces animal feeds, and can be used to produce other products.
2. Description of the Background
Currently most fuel ethanol produced in the U.S. is made from corn grain. Even if all the corn grain produced in the US were converted to ethanol, it would only supply about 15% of our current transportation fuel needs. Thus, there is a pressing need to produce fuel ethanol and other alcohols from other sources of feedstock. If ethanol could be inexpensively produced from plant fiber, waste biomass like leaves, paper, manure, wood byproducts, and others materials, it could offset fuel shortages. Plant fiber, also called cellulosic biomass, can be grown on marginal land and in greater yields than grain crops. Eventually, the U.S. aims to use up to a billion tons of such biomass per year. Other waste biomass includes garbage comprised of waste plastic or other forms of fossil fuel derivatives.
Plant fiber is also called plant cell wall, which is comprised of cellulose, hemicellulose, pectin, and lignin. There are a few processes available for the production of ethanol from plant fiber. One process is physical conversion: biomass is heated to high temperatures, such as 650° F. The biomass is degraded to carbon monoxide (CO) and hydrogen (H2), and subsequently these gases are converted to ethanol by a catalytic or microbial process. The advantage of this approach is that many forms of biomass or fossil fuel derivatives can be used, but the cost of facilities may be high compared to anaerobic digestion. In addition, waste gases from other industrial processes can be used, or even gases produced by anaerobic digestion can be efficiently used.
Use of microorganisms to produce acetic acid or ethanol from CO2, CO and H2 was disclosed in U.S. Pat. No. 5,173,429; U.S. Pat. No. 5,593,886; and U.S. Pat. No. 6,136,577, which are incorporated herein by reference. However, the ratio of acetic acid to ethanol was 20:1 or greater and only 0.1% ethanol concentration could be achieved. In U.S. Pat. No. 7,285,402, incorporated herein by reference, ethanol concentrations greater than 10 g/L and acetate concentrations lower than about 8-10 g/L were claimed, while continuing to permit culture growth and good culture stability. However, the cost of achieving these rates through physical manipulations of the fermentation, and the cost of distillation for such low concentrations of ethanol would be cost prohibitive for an industrial process.
A second approach is called biochemical conversion: the biomass is boiled in caustic acids or other chemicals to hydrolyze the cellulose and hemicellulose. The residue is neutralized and conditioned and subjected to cellulolytic enzymes to release sugars. The glucose released is fermented by yeast to ethanol, and the 5-carbon sugars are separated and converted to ethanol by a separate organism.
A third approach to producing cellulosic ethanol would be to use living microorganisms that can digest cellulose, hemicellulose and pectins and convert them to ethanol. This approach would be least expensive because it does not require harsh chemicals or high temperatures and uses fewer processing steps. However, the approach is only feasible if there is a microorganism, or mixed culture of microorganisms, that can readily digest cellulose and hemicellulose, and which, preferably converts a significant part of the carbohydrate to ethanol. The ideal organisms would also be tolerant to ethanol concentrations so that they can be used to digest considerable carbohydrate to ethanol at high enough concentration to decrease the cost of distillation.
Microorganisms can be used for aspects of all three processes. In the first case, microorganisms can assimilate the synthesis gases, such as CO2, CO and H2 into ethanol or acetic acid, or into longer chain alkyl alcohols (e.g. 1-propanol, 1-butanol) or longer chain carboxylic acids (e.g. propionate, butyrate). In the second case, organisms are used to produce enzymes for the degradation of plant fiber and for fermentation of sugars into ethanol. In the third case, microorganisms are used to both digest plant biomass and convert it to alcohols. Finally, microbial cultures that can both digest biomass (case 3) and assimilate gases into alcohols (case 1) can be used. In this case, the gases that are produced by organisms in the digestion of the biomass can be converted to ethanol or other alcohols.
For either the first or the third process, or a combination thereof, two desired characteristics of the microorganisms used are: 1) ability to convert a large portion of the substrate (e.g. gases or biomass) to the desired products (e.g. alcohols or acids), and 2) ability to continue producing the desired product even in the presence of high concentrations of those products. Currently, microorganisms are not available for conversion of synthesis gases to high concentrations of alkyl alcohols. The ability to tolerate high concentrations of products, and to still produce more of the product at high concentrations (about 5% to about 6%, by volume), would make it possible to produce the products in a way in which it is cost effective to separate and utilize the products.